Broadcast profile updates so member lists stay live (no reload needed)

Another user's new display name or avatar didn't show up until you
reloaded -- update_profile/upload_avatar/remove_avatar never told
anyone. Same root cause and fix shape as #26 (room_added): the
frontend's already-fetched member list had no way to hear about a
change, since nothing ever pushed one.

Reuses the existing per-room broadcast channel (not the per-user one
#26 added, since this only matters for rooms the affected user shares
with someone currently looking at them) -- publishes member_updated to
every room the user belongs to; ChatShellPage refetches members when
it arrives for the currently open room.

Verified end-to-end in the browser: one user's room-info member list
updated live when another user changed their display name from a
separate session, no reload.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/ChatShellPage.tsx

@@ -64,8 +64,9 @@ export function ChatShellPage() {
     () =>
       socket.subscribe((envelope) => {
         if (envelope.type === 'room_added') refreshRooms()
+        else if (envelope.type === 'member_updated' && envelope.room_id === roomId) refreshMembers()
       }),
-    [socket, refreshRooms],
+    [socket, refreshRooms, refreshMembers, roomId],
   )
 
   useEffect(() => {

frontend/src/types.ts

@@ -119,6 +119,12 @@ export interface ChatRoomAddedEnvelope {
   room_id: string
 }
 
+export interface ChatMemberUpdatedEnvelope {
+  type: 'member_updated'
+  room_id: string
+  user_id: string
+}
+
 export type ServerEnvelope =
   | ChatMessageEnvelope
   | ChatMessageUpdateEnvelope
@@ -126,6 +132,7 @@ export type ServerEnvelope =
   | ChatJoinedEnvelope
   | ChatErrorEnvelope
   | ChatRoomAddedEnvelope
+  | ChatMemberUpdatedEnvelope
 
 export interface AdminUser {
   id: string